1. Field of the Invention
The present invention relates to a method of generating gradation-corrected image data, a method of generating gradation-corrected halftone dot percentage data, a method of generating gradation-corrected threshold data, an image output apparatus, and a method of calculating amount of exposure, for preventing a gradation jump or tone jump on an outputted image, suitable for use in an image output system such as a printing scanner for outputting a film with a halftone-dot image formed thereon, a CTP (Computer To Plate) output system for outputting a printing plate with a halftone-dot image formed thereon, or a color printer for outputting a hard copy which carries an image according to density gradation principles.
2. Description of the Related Art
One example of the cause of a gradation jump or tone jump will be described below.
An image reading/outputting apparatus, for example, as a halftone dot generator photoelectrically reads the image information of a subject with an image sensor to produce gradation image data (also referred to as multivalued image data or continuous image data) and thereafter compares the gradation image data with threshold data to generate binary image data (also referred to as halftone-dot image data).
The image reading/outputting apparatus applies a laser beam, which is turned on and off, i.e., modulated, by the binary image data, to scan a film coated with a photosensitive material for thereby producing a film on which a halftone-dot image is formed.
The halftone-dot image formed on the film is an area-gradation image whose gradation is expressed by forming black pixels such that halftone-dot area percentages will be obtained depending on the gradation of the gradation image data.
FIG. 16 of the accompanying drawings schematically shows a halftone-dot image 1 whose halftone-dot area percentage is 50%. As shown in FIG. 16, the halftone-dot image 1 has. black pixels 3 in regions 2a, 2b which contact each other at a point 4. The black regions 2a, 2b also contact black regions of other halftone-dot images (not shown) each having a halftone-dot area percentage of 50%.
At the points 4 where the black regions contact each other, the laser beams for forming the pixels 3 overlap each other. Therefore, each of the points 4 tends to spread, and fails to achieve an accurate halftone-dot area percentage of 50%. Instead, each of the points 4 suffers a tone jump as a gradation distortion when the images are reproduced.
Such a halftone-dot image is formed on a film as a negative film by a film producing process, which is then converted to a positive film by a film converting process. From the positive film, there is produced a pressing plate by a printing plate producing process, which will be used in a printing process to produce a printed document.
If no accurate halftone-dot area percentage is accomplished due to a tone jump, then the tone jump will also be produced in not only the film producing process, but also the converting process, the printing plate producing process, and the printing process.
FIG. 17A of the accompanying drawings shows the relationship between gradation image data and halftone-dot area percentages in the film producing process. FIG. 17B of the accompanying drawings shows the relationship between halftone-dot area percentages on a film and halftone-dot area percentages on a positive film in the film converting process. FIG. 17C of the accompanying drawings shows the relationship between halftone-dot area percentages on a positive film and halftone-dot area percentages on printing plate in the printing plate producing process. FIG. 17D of the accompanying drawings shows the relationship between halftone-dot area percentages on a printing plate and halftone-dot area percentages on a printed document in the printing process. FIG. 17E of the accompanying drawings shows the relationship between gradation image data and halftone-dot area percentages on a printed document, the relationship being representative of an accumulation of the relationships shown in FIGS. 17A through 17D.
The relationship between outputted halftone-dot area percentages and desired halftone-dot area percentages differs depending on output machine conditions, halftone attributes, etc., e.g., whether a halftone-dot image is to be outputted to a film as a recording medium by a laser beam, whether a negative film is to be converted to a positive film, whether a printing plate is to be produced, whether a halftone-dot image is to be printed with ink, and other conditions including a screen ruling, a screen angle, a halftone dot shape. As shown in each of FIGS. 17A through 17E, if the relationship between input data and output data is nonlinear, then the nonlinear relationships in the various processes ranging from the film producing process to the printing process are accumulated, as shown in FIG. 17E, resulting in a tone jump that is greatly noticeable to view.
Prior attempts to prevent such tone jumps include a method of correcting halftone-dot image data and an image processing apparatus with a correcting function as disclosed in Japanese laid-open patent publication No. 6-303429, an image data processing apparatus as disclosed in Japanese laid-open patent publication No. 7-336536, and an image data processing apparatus as disclosed in Japanese laid-open patent publication No. 8-98025.
According to the disclosed image data processing techniques, in an image output process up to a desired process for preventing a tone jump, an image carrying a plurality of test patches is outputted as a reference corresponding a plurality of halftone-dot area percentage data, the test patches are measured by a densitometer or a halftone-dot area meter, and a corrective curve is determined on the basis of the measured values. Using the corrective curve, the gradation of inputted gradation image data or threshold data is corrected in order to produce correct output gradations.
The above disclosed image data processing techniques are advantageous in that they are capable of outputting halftone-dot images having gradation characteristics free of tone jumps.